[Rails]ActiveRecord::Migration - 改變欄位資料型態

來源：http://stackoverflow.com/questions/2799774/rails-migration-for-change-column

Step 1:

Generate a new migration file using this code:

rails g migration sample_name_change_column_type

Step 2: 

Go to /db/migrate folder and edit the migration file you made. There are two different solutions.

1. def change
       change_column(:table_name, :column_name, :new_type)
   end

2.

    def up
        change_column :table_name, :column_name, :new_type
    end

    def down
        change_column :table_name, :column_name, :old_type




    end

Step 3:

Don't forget to do this command:

rake db:migrate

實際操作：
在command 下，新增migration 檔案

➜  eat git:(master) ✗ rails g migration sample_name_change_column_type

Running via Spring preloader in process 74424

      invoke  active_record

      create    db/migrate/20161008204016_sample_name_change_column_type.rb

在sublime text 開啟

自行修改欄位，依序是資料表名稱、欄位名稱、新的資料型態。

在此示範的是將 reviews資料表裡的rating欄位改成float 資料型態。
 change_column(:table_name, :column_name, :new_type)

在command 下

➜  eat git:(master) ✗ rake db:migrate

== 20161008204016 SampleNameChangeColumnType: migrating =======================

-- change_column(:reviews, :rating, :float)

   -> 0.0064s

== 20161008204016 SampleNameChangeColumnType: migrated (0.0065s) ==============

在rails c 下測試
可以看到紅色處由integer改為float

irb(main):002:0> a = Review.last

  Review Load (0.2ms)  SELECT  "reviews".* FROM "reviews" ORDER BY "reviews"."id" DESC LIMIT ?  [["LIMIT", 1]]

=> #<Review id: 14, rating: 3, comment: "3.5", created_at: "2016-10-08 08:15:48", updated_at: "2016-10-08 08:15:48", user_id: 3, store_id: nil>

irb(main):003:0> a.rating.class

=> Fixnum

irb(main):004:0> a.rating.class

=> Fixnum

irb(main):005:0> exit

➜  eat git:(master) ✗ rails c

Running via Spring preloader in process 74494

Loading development environment (Rails 5.0.0.1)

airb(main):001:0> a= Review.last

  Review Load (0.2ms)  SELECT  "reviews".* FROM "reviews" ORDER BY "reviews"."id" DESC LIMIT ?  [["LIMIT", 1]]

=> #<Review id: 14, rating: 3.0, comment: "3.5", created_at: "2016-10-08 08:15:48", updated_at: "2016-10-08 08:15:48", user_id: 3, store_id: nil>

irb(main):002:0>